Identification of Product Chemistry, Properties, Uses,

World Market and Projected Market GrowthAcetic acid propertiesAcetic acid is a clear colourless liquid that is very corrosive and has anoticeable odour. The odour can be detected when concertation is as lowas 1ppm. A table summarising the physical properties is below;Acetic AcidAppearance

Perrys chemical Engineering handbook Antoine Constants for Cp

(J/Kmol)SpecificHeatCapacitiesComponent

c1

c2

c3

Acetic Acid139640-32.80.923Using equation Cp= C1 +C2T+C3T + C4T +C4T4

c4

c5

Methanol Carbonylation (Monsanto process)

Acetic Acid manufactured via carbonylation of Methanol Monsanto processuses a homogeneous catalyst which is usually a Rhodium-based catalyst.The main reactions that take place inside the reactor and in the presenceof the catalyst are as follows:

CH3OH + HI CH3I + H2O

CH3I + CO CH3COICH3COI + H2O CH3COOH + HI

(1)(2)(3)

The process characteristically takes place at a high pressure and

temperature which are in the region of between 10-30 bar and 175-200C.Using a Methyl Iodine (CH3I) Rhodium complex as a catalyst is the mostcommon type catalyst used for this process as it achieves a very highconversion rate of around 99% from the methanol at the temperaturesand pressures discussed above. A study of the reaction kinetics showsthat the methanol reaction is a zero order reaction while the rhodium andiodide promoter are both first order reactions. The most dominant sidereaction that occurs in this process, is the production of methyl acetatewhich can be a valuable side product if extracted and separated from themain product or can be easily recycled to drive down running costs. Theproduction of methyl acetate is equilibrium limited and varies andchanges with any temperature or pressure change.. The reaction thatoccurs to produce methyl acetate is as follows:CH3OH + CH3COOH CH3COOCH3 + H2O (4) (esterification reaction)

An assumption can be made that all methyl acetate that does make itback into the reactor via the recycle streams will react with any of thewater that is and that around 50% of the unreacted methanol leaving thereactor then reacts to form methyl acetate.Other by products produced during the process are; dimethyl ether,methyl acetate, acetoaldehyde, butyraldehyde, ethyl acetate andpropinoic acid.

(Ullmanns, 2003, p. 159)

a)b)c)d)e)

ReactorFlasherLight ends columnDehydration columnHeavy ends column

To begin the process Methanol and compressed Carbon monoxide (30 barto ensure the reaction occurs in the liquid phase) enter a sparged tubularreactor under the reaction conditions mentioned above (10-30 bar andbetween 175-200C) in order for the reaction to occur. The reaction ishighly exothermic and therefore a cooling mechanism for the rector suchas a cooling jacket is needed to ensure a stable temperature is kept andthe excess heat is removed. The hydrocarbon vapour stream exiting thetop of the reactor is mainly composed of unreacted gasses and can beeasily recovered using a scrubber for recycling. The resultant reactedmixture (vapour phase) is then expanded by the use of a flasher or turbineto recover duty to compress air for use in the reactor and to also recoverany of the catalyst which is then sent back to the reactor. The flashercondenses the hydrocarbons and the light hydrocarbons are thenseparated by the subsequent column to a cut point of 80C and eitherrecycled or sent to storage for further treatment and separation. The lowboilers that are separated are dimethyl ether, methyl acetate,acetaldehyde, butyraldehyde and ethyl acetate. The Remaining mixturefrom the column along with the main product is then sent through to thedehydration column (d), in which the top organic layer taken from thecolumn is very rich in hydrocarbons and is sent back to the reactor forrecycling. The bottoms aqueous layer from the dehydration column isdistilled to recover the hydrocarbons and then also recycled back to thereactor. The remaining hydrocarbon free product consists of volatileoxygenated derivatives (aldehydes, ketones, esters and alcohols), water,volatile monocarboxylic acids (formic, acetic, propinoic and butyric frombutane), and a mixture of non-volatile materials (difunctional acids,butyrolactone, condensation products, catalyst residues etc.). The volatilesubstances mentioned above can then be further recovered as mixtures orindividual and sold as by-products or recycled back to the reactor to drivethe running costs down. Most of these volatile products that wereseparated can generate acetic acid on further oxidation and treatment.The separation of water and formic acid from the acetic acid productmixture involves the use of several distillation columns. Water removal isdifficult and very costly when compared to the rest of the purificationprocess it is carried out by azeotropic distillation with entrainment agentssuch as ethers. Formic acid is then separated from the resultinganhydrous acetic acid by again further distillation with an azeotropingagent which will result in a mixture of higher boiling acids and acetic acid.These remaining higher boiling acids are then separated from the aceticacid product as a residue stream from the bottom of the tower bydistillation. An option to add hydrocarbons such as heptane and isooctane6

to the reactor to improve separation by forming a high boiling azeotrope

with formic acid exists that might help drive the plant running andequipment costs down. The non-volatile residue can usually be burned offto recover energy to heat other parts of the process. Traces of iodine thatmay remain in the product acetic acid produced may be removed toincrease the purity of the acid by fractional crystallisation or by addition ofsmall amounts of methanol followed by distillation of the methyl iodidethat subsequently forms. A small amount of propionic acid is also madeand is typically found in the residue of the acetic acid finishing system andcan be removed from the residue by purging the column bottoms and canbe a viable product on its own.